This invention relates to the polymerization of water-miscible monomers in aqueous solution.
Most water-soluble polymers, particularly polymers of acrylamide, acrylic acid and their water-miscible derivatives have been employed commercially as additives in the manufacture of paper products, as water purification coagulants, as dispersing agents, and as treating agents in a wide variety of applications.
Of the various methods employed to polymerize water-miscible monomers, the aqueous solution polymerization method in a batch mode is most commonly employed because it is inexpensive and can provide water-soluble polymers having high molecular weight. In such a method, the concentrations of monomer and resulting polymer in aqueous solution are maintained as high as possible in order to reduce the amount of water that is subsequently removed from the resulting polymer product. Unfortunately, the high viscosities of the resulting polymer solution, even at fairly low conversion, limit the initial monomer concentration to below a 2 or 3 weight percent concentration. This concentration limit leads to poor reactor utilization and the high viscosity leads to poor reactor heat transfer due to the inability to stir the contents.
The problems that result from the high viscosities of relatively dilute aqueous solutions of such water-soluble polymers have been solved by suspending or emulsifying the aqueous solution of water-miscible monomer in a water-immiscible organic liquid and forming a relatively unstable suspension or a relatively stable emulsion of the desired water-soluble polymer. Methods of practicing such suspension or emulsion polymerization are described in U.S. Pat. Nos. 2,982,749 and 3,284,393, respectively. These methods significantly increase the concentration of monomer and resulting polymer in the suspension or emulsion as compared to the amount of monomer or polymer present in the aforementioned batch techniques while maintaining workable viscosities. Unfortunately, the cost incurred in employing such processes is high due to the use of organic solvents which are not recovered or reused. It has also been difficult to practice such methods in a continuous manner.
Water-soluble polymers can be prepared in a continuous manner using a tubular reactor as described in U.S. Pat. No. 4,110,521. Static mixers are used to promote plug flow and heat transfer while running the polymerization isothermally. The continuous tubular reactors enjoy cost and utilization advantages over batch processes but, unfortunately, such processes require costly temperature control systems, costly static mixing elements, and high pressures to force highly viscous gels through the system. U.S. Pat. No. 4,331,787 offers a partial solution to the requirement of high pressure that forces the resulting polymer through the system by employing a water-immiscible fluid to lubricate the flow of emulsified polymer gels during continuous tubular polymerization. Unfortunately, such a process still requires pressure to force the polymer through the system, and the addition of a lubricant can contaminate the polymer product and add to processing costs.
In view of the aforementioned deficiencies of the prior art methods, it is highly desirable to provide a continuous process for polymerizing water-miscible monomers to form water-soluble polymers that have high molecular weights, which process comprises a compact, simple, self-contained and energy efficient system without the use of water-immiscible solvents and lubricants.